A power-on reset (POR) generator is a microcontroller or microprocessor peripheral that generates a reset signal when power is applied to the device. A POR ensures that the device starts operating in a known state. Typically in very large-scale integration (VLSI) devices, where the current level of computer microchip miniaturization is complex and refers to microchips containing in the hundreds of thousands of transistors, the POR is an electronic device incorporated into the integrated circuit that detects the power applied to the chip. The POR generates a reset impulse to the entire circuit placing the circuit of affected chip into a known state.
A typical power switch consists of a large P-channel Metal Oxide Semiconductor (PMOS), a buffer, and a level-shifter circuit.
A simple POR comprises an RC device that charges with the rising of the supply voltage. It is understood to use a Schmitt trigger such that the rising charged voltage of the RC network generates an impulse. This impulse is generated based on the two threshold voltages of the Schmitt trigger. When the input voltage at the Schmitt trigger coming from the RC network reaches the first threshold voltage the output of the Schmitt trigger switches so that it generates the first edge of the input. The charging of the RC network should be long enough so that the POR can reset all the internal circuits before the charging voltage reaches the other threshold voltage of the Schmitt trigger and the output to switch back.
FIG. 1 depicts a typical POR circuit 12 as known in the prior art. In the circuit of FIG. 1, Vcc voltage 2 represents the voltage from a power supply which is not shown and POR 10 represents the power-on reset signal which is generated by a Schmitt trigger 8. Before the power supply is turned on, the POR 10 is low since capacitor 6 is discharged and since the Schmitt trigger 8 does not have any source of power (since it is also powered by the power supply). When the power supply is turned on, the Vcc voltage 2 rises and begins to charge capacitor 6 through resistor 4 and power the Schmitt trigger 8. Until the Schmitt trigger 8 has sufficient voltage to operate, its output is in an indeterminate state. When the voltage on capacitor 6 reaches the threshold voltage of the Schmitt trigger 8, the output of the Schmitt trigger changes to a high state and thus the POR signal 10 swings to a high state.
Prior art power-on reset circuit 12 in FIG. 1 is limited to situations where Vcc has a rise time at turn-on that is much faster than the RC time constant. As a result, large resistors and capacitors are required for those power supplies which have long rise times. The large capacitors can use a significant amount of area on an integrated circuit. Additionally, the POR output is indeterminate at VCC voltages below the operating voltage of the Schmitt trigger 8.
One task of the traditional POR is ensuring that the processor starts at a known address when power is first applied. To accomplish that task, the POR logic output holds the processor in its reset state when the processor's power supply is first turned on. Typically, the POR's second task is to keep the processor from starting its operation from that known address until three events have occurred: (1) the system power supplies have stabilized at the appropriate level; (2) the processor's clock(s) have settled; and (3) the internal registers have been properly loaded.
System designers readily embrace a traditional POR and in particular their designs often involve providing for disabling the microprocessor with a POR until the power supply has reached a sufficient and stable voltage for the microprocessor to operate reliably. Therefore, by disabling the microprocessor with a power-on reset circuit until the power supply provides a known good voltage, errors in the microprocessor due to low voltage and/or noise are avoided.
While a POR is recognized to be a good voltage source necessary for powering on to reset synchronous circuitry, unfortunately, a POR only provides an impulse at a particular time and is unable to provide other inputs to a circuit, such as an externally controlled voltage for powering-up other components in the circuit or for providing power through switching during a transition period between ON and OFF functions. Examples of devices which benefit from traditional POR activities include microprocessor-based system such as a personal computer, an automobile, or a radio, for example. Many devices in use today, consumer and non-consumer type products, which use electronics, often employ a traditional POR to provide enabling of the operation of the electronics when a reliable power source is available and disabling the operation of the electronics when the reliable power source is not available.
Accordingly, what is needed is a Power-on-reset (POR) circuit implemented in a power switch to enable the switch at a predefined voltage and to power-up successive integrated circuits after initially energizing a first circuit without over-current or over-voltage effects in the circuit.